1. Field of the Invention
The present invention relates to a single-mode optical fiber used as a transmission line in optical communications or the like; and, in particular, to a dispersion-shifted optical fiber suitable for wavelength-division multiplexing (WDM) transmission.
2. Related Background Art
Conventionally, in optical communication systems employing single-mode optical fibers as their transmission lines, light in the wavelength band of 1.3 .mu.m or 1.55 .mu.m has often been utilized as signal light for communications. Recently, among others, the use of light in the wavelength band of 1.55 .mu.m has been increasing from the viewpoint of reducing transmission loss in transmission lines. Single-mode optical fibers employed in such transmission lines for light in the wavelength band of 1.55 .mu.m have been designed such that their wavelength dispersion (phenomenon in which pulse waves are broadened due to the fact that the propagation speed of light varies depending on wavelength) with respect to light in the wavelength band of 1.55 .mu.m is nullified (to yield dispersion-shifted optical fibers having a zero-dispersion wavelength of 1550 nm).
Also, as long-haul transmission has become possible with the advent of WDM transmission or optical amplifiers in recent years, in order to reduce nonlinear phenomena (to suppress distortion of signal light), a dispersion-shifted optical fiber of a dual-shape core structure having an effective area A.sub.eff of 70 .mu.m.sup.2 or more, such as that shown in FIG. 1A, has been proposed (Japanese Patent Application Laid-Open No. 8-248251). Here, nonlinear optical effects refer to phenomena in which signal light pulses are distorted in proportion to density of light intensity or the like due to nonlinear phenomena such as four-wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), and the like, thereby restricting transmission speed or repeater spacing in relay transmission systems.
In such a dispersion-shifted optical fiber, as shown in FIG. 1B, the refractive index profile of its core region 10 is designed such that the power distribution PA(r) of propagating light attains its maximum value PA.sub.max at the center position O.sub.x (r=0) in the core region 10. Further, at the outer periphery of the center part of the core region 10 having a higher refractive index, a peripheral area having a lower refractive index is provided. The introduction of peripheral area broadens the envelope of optical power distribution PA(r). Since the envelope is broadened, the effective area A.sub.eff becomes 70 .mu.m.sup.2 or more (so as to prevent the optical power from being concentrated at the center of the core), thereby reducing the above-mentioned nonlinear phenomena.
In the conventional dispersion-shifted optical fiber shown in FIG. 1A, the center part of the core region 10 with a higher refractive index has an outside diameter of 4.1 .mu.m, and a relative refractive index difference of 0.94% with respect to its cladding region 20. The peripheral area of the core region 10 with a low refractive index has an outside diameter of 31.5 .mu.m, and a relative refractive index difference of 0.20% with respect to the cladding region 20. When the refractive index profile of the core region 10 is thus designed, its effective area A.sub.eff becomes 70 .mu.m.sup.2 or more.